The present invention relates to an electro-hydraulically controlled transmission, and in particular to a transmission having one or more releasable coupling mechanisms, such as a clutch or a brake, wherein only one such mechanism needs to be engaged for a transmission output shaft to be drivingly coupled to the input shaft. A two-step actuator valve arrangement is provided in the hydraulic control circuit so that two valves must be moved to open positions to engage a releasable coupling mechanism.
It is common in vehicle transmissions, such as a power shift transmission, to have a number of clutches and/or brakes that are selectively engageable in various combinations to route power through the transmission to produce different gear ratios. In a typical power shift transmission, multiple clutches and/or brakes must be engaged to provide power to the transmission output shaft. To engage multiple clutches or brakes, multiple valves in the hydraulic circuit need to be opened. Multiple valves are needed because different combinations of brakes and clutches must be engaged to produce different transmission gear ratios. A dedicated valve controls each clutch and brake. The need for multiple valves to be opened to provide power to the output shaft significantly reduces the likelihood of inadvertently doing so due to a valve sticking in an open position.
The present invention is useful in a transmission where only a single releasable coupling mechanism, such as a clutch or brake, needs to be engaged to provide power to the output shaft. The present invention provides a two-step actuator valve arrangement in which two valves arranged in series must be opened before any releasable coupling mechanism is engaged to power the output shaft. In the present invention, a single first-step actuator valve is arranged in series with a plurality of second-step actuator valves. The second-step actuator valves are arranged parallel with one another. In the resulting control circuit, both the first-step actuator valve, and one of the second-step actuator valves, must be opened before power is provided to any one of the clutches.